Can anyone think of a reason why it would be a problem to perform a diacetyl rest AFTER primary fermentation, secondary fermentation, and a period of lagering have occurred? My idea is that after lagering for about 3 weeks, I would warm the beer up to about 55 degrees and add one or two fresh Activator packs. My assumption is that the fresh yeast should mop up any residual by-products (diacetyl, acetylaldehyde) that the primary and secondary fermentations did not reduce. After this fresh yeast had done its work (maybe 5 days or so), I would crash cool back to lagering temps and finish lagering for another month or so, flushing any flocculent yeast from the conical as necessary.

I know that this may seem like a waste of money for the additional yeast, but I don't like the idea of (1) having to catch the fermentation at exactly the right time in order to warm it up for a diacetyl rest (because this risks either warming it up too early and generating undesirable by-products or warming it up too late and achieving an insufficient clean-up) and (2) I wonder if the yeast that are still in suspension towards the end of fermentation are sufficiently viable and numerous to perform a successful clean-up.

My main concern about this technique is that, lacking additional fermentables and being pitched into a high-ethanol environment, the fresh yeast would be like "screw you guys, I'm going home" and would flocculate to the bottom without doing any work.

I would just plan to prevent it in the first place. If you pitch cold, pitch the correct amount of yeast, and oxygenate well, you will more than likely not need a diacetyl rest.

Also, there is not an exact right time to catch the fermentation, any increase in temp towards the end of fermentation will encourage the yeast to uptake the diacetyl. This is only to increase their rate of uptake really, if you let it sit long enough the yeast will re-absorb it without the increase in temperature.

I don't think fresh yeast are going to clean up diacetyl after primary fermentation is complete. However, you can do this if you like... you just need to "krausen" the beer. I.e., make a yeast starter with your new yeast, ideally with wort from brewday. You can pitch it when it is at high krausen any time after fermentation is complete and it will help clean up diacetyl and other compounds. IIRC, the proper volume for krausening is 1/10 the volume of the beer.

Ok, I was assuming that the yeast would have to be in an active state (i.e. not in their stationary phase where little metabolic activity is occurring) in order to clean up diacetyl. I guess I need to read up on eactly what process of metabolism is involved in converting diacetyl and acetylaldehyde.

Regarding krausening, I was thinking of that as well, because it is essentially the same idea, except that new fermentables are being added to the beer along with the yeast. My only concern -- again, probably a misguided concern based upon a misunderstanding of the process -- is that this second fermentation may require a period of conditioning to clean up after itself. It makes me wonder if, apart from the inconvenience and additional cost, it wouldn't ALWAYS be a good idea to krausen -- at least, if the goal is reducing diacetyl to a minimum.

I don't know any situation (at least for a lager) where krausening would be a bad idea. It is supposed to clean up and smooth out the beer. I've never done it myself (i.e., actual krausening by definition), but Greg Noonan speaks highly of it in New Brewing Lager Beer.

Just set aside 1/10 the volume of the wort on brewday... put it into canning jars and boil it in the jar for like 10 minutes. Then, you can keep and wash yeast from primary, add it to the canned wort in a starter-sized vessel. This way you don't need any extra yeast or wort.

This secondary fermentation doesn't need to clean up after itself. It is the cleaner-upper.